1. Field of the Invention
This invention relates to an ignition system for internal combustion engines for vehicles.
2. Description of the Prior Art
In view of the recent problem of air contamination, it is very important to reduce the content of harmful components in the automotive exhaust gas for reducing the contamination. To give a solution to this problem methods of recirculating part of the exhaust gas from the internal combustion engine to the intake system thereof have been investigated. Also, it is a recent trend to use lead-free gasoline incorporating high-molecular compounds in place of lead in order to extent the service life of the catalyst used for purifying the exhaust gas.
It will be seen from the above that with the progress of the measures against the exhaust gas the condition for the combustion of the air-fuel mixture in the engine cylinder becomes severer, that is, increased energy and prolonged spark duration are required for bringing about the ignition. Also, where high-molecular compounds are incorporated in the fuel and where the exhaust gas is recirculated, the spark plug is more likely to be contaminated with carbon and other deposits. These deposits form shunt resistance between the discharge points of the spark plug, thus reducing the voltage applied across the spark plug gap.
Accordingly, the ignition system is required to meet the following:
1. It has to be superior in its ability to withstand contamination.
2. It must have a superior combustion ability.
3. The spark duration should be long.
As a further basic requirement for the ignition system, it is desired to be able to take out a constant spark energy irrespective of the engine speed. With a usual transistor type ignition system the energy stored in the primary winding of the ignition coil varies to vary the spark energy with the engine speed.
Still further, a spark of great energy is desired in the initial stage of the spark discharge. This is because a great spark energy is required for the ignition of the air-fuel mixture where the ratio of fuel to air is large.
Heretofore, no ignition system meeting all of the above requirements has existed.
In a typical prior-art ignition system, a predetermined current carried by the primary winding of the ignition coil is cut off in accordance with the ignition timing, thereby bringing about the discharge in the spark plug gap due to the energy stored in the primary winding. The cutting off the primary winding current is usually effected by means of a transistor or a mechanical switch. (This system is hereinafter referred to as a transistor type ignition system). In another system, a capacitor is charged with a stepped-up high voltage and is discharged through the primary winding of the ignition coil in accordance with the ignition timing. In this case, the discharge of the capacitor through the primary winding is effected through a thyristor. (This system is hereinafter referred to as a thyristor type ignition system.)
As mentioned above, in a transistor type ignition system the engine speed versus spark energy characteristic is inferior, that is, the spark energy varies greatly with the engine speed. Also, since the primary side of the ignition coil is opened when the spark is produced, the effect of the resistance of the secondary side of the ignition coil and the energy loss due to the contamination of the spark plugs are great.
In a thyristor type ignition system, the primary side of the ignition coil is short-circuited at the time of generation of a spark, so that this system is less affected by the contamination of the spark plugs. Also, since the energy stored in the capacitor is supplied to the primary winding of the ignition coil in a short time, a very large energy can be obtained momentarily. However, in this system the spark duration is very short. Therefore, it is very difficult to ensure ignition where the proportion of fuel is very small.
Accordingly, there is a need for an ignition system, in which the above drawbacks inherent in the prior-art ignition systems are minimized.